There is a continuing demand for advanced gas turbines that achieve lower fuel burn rates and commensurate reduced carbon dioxide exhaust emissions. Therefore, there remains a pressing need for superalloys that can be cast into complex cooled turbine blades and vanes that are capable of operating at higher gas and metal temperatures. These castings desirably exhibit a combination of high strength, excellent high temperature, creep-rupture properties, and good phase stability.
Single crystal nickel-base superalloys typically contain high levels of refractory elements such as molybdenum, tungsten, rhenium and tantalum in order to improve high temperature creep-rupture properties. However, high levels of these elements can result in topologically close-packed (TCP) phase formation during high temperature stressed exposure, which can be associated with the development of sites for premature crack initiation, resulting in a degradation of long term creep-rupture properties. As such, the selection of appropriate levels of refractory elements and chromium content involves a delicate balancing of strength properties against long term phase stability. The TCP phases are rhenium and tungsten rich with some chromium. Excessive formation of TCP phases de-alloy the material, thus lowering the creep-rupture strength.
The highest strength nickel-base superalloys for single crystal castings for use in flight engines contain about 5% to about 7% rhenium by weight. These include CMSX-10K® and CMSX-10N® alloys, developed and available from Cannon-Muskegon Corporation and described in U.S. Pat. Nos. 5,366,645 and 5,540,790, and Rene N-6 alloy, developed by the General Electric Company. However, these specialty, high-strength nickel-base superalloys have exhibited certain undesirable features for particular applications. These alloys tend to develop a type of phase instability due to the high rhenium-content, which is known as a secondary reaction zone (SRZ) instability, that is observed in the base alloy adjacent to the coatings, which results in coating compatibility and thin-wall mechanical property issues. Additionally, CMSX-10K® and CMSX-10N® alloys have a very low chromium content (1.5% and 2.2% by weight respectively) to accommodate the high rhenium content, which consequently reduces low temperature internal oxidation resistance and hot corrosion resistance. These alloys also have high γ′ solvus temperatures, requiring a very high temperature solution heat treatment, which can cause surface melting issues. These alloys also tend to have a relatively high density, which is a significant weight and inertia disadvantage for flight engines, especially for rotating turbine blades. These very high strength specialty alloys are also expensive due to the high price of rhenium, which has approximately quadrupled in the last 20 years.